Temperature control is important in many manufacturing processes such as wafer processing. Typically, a load temperature must be controlled within the tolerance of a setpoint, which represents a user-defined ideal value for the load temperature.
Generally speaking, the load temperature may be controlled by controlling the amount of power provided to an electrical heater. The heater is typically controlled by selectively turning on and turning off the electrical power provided from an AC power source to the heater using, for example, a solid state relay (SSR) circuit. Conventionally, the SSR may be controlled by a pulse width modulation (PWM) signal, which is provided to the SSR responsive to a duty cycle command provided by a controller.
The duty cycle command represents the desired percentage of the time that the SSR must turn on the power supply, such that the desired amount of power is supplied to the heater within a given time duration to ensure that the load temperature is within the tolerance of the setpoint. In order to comply with the required tolerance of a given manufacturing process, the duty cycle command typically needs to have a certain level of resolution (or precision). For example, the tolerance may require the duty cycle command to have a resolution of 0.1%. Accordingly, for example, a duty cycle command provided as 65.8% complies with the required tolerance whereas a duty cycle specified simply as 65% does not comply with the required resolution of 0.1%.
An example conventional method for controlling the heater with a required duty cycle command resolution is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a flowchart 100 of the example conventional method for controlling a heater 135. FIG. 1B shows a corresponding a block diagram 120 of an example conventional system for implementing the example conventional method of FIG. 1A.
The example conventional method starts with step 102, in which a summing function 124 receives a setpoint 122 (see FIG. 1B). Setpoint 122 is typically expressed in degrees of temperature, which may be in Celsius, Kelvin, or Fahrenheit. In step 104, summing function 124 compares a load temperature 136, which may be sensed using, for example, a temperature sensor, with setpoint 122 and outputs a temperature error, i.e., the difference between load temperature 136 and setpoint 122. In step 106, a controller 126 takes into account the setpoint as well as the temperature error and generates a duty cycle command, e.g., 65.8235%.
Next, in step 108, hardware 132 quantizes the duty cycle command into a quantized duty cycle command, e.g., 65.8% with a required resolution, e.g., 0.1%. In step 110, based on the quantized duty cycle command, hardware 132 determines a number of ‘on’ AC cycles, i.e., a number of AC cycles that the power supply should be turned on. Since the required resolution is 0.1%, 1000 AC cycles are required to permit discrimination to 0.1% precision or 1 out of 1000 cycles. In the current example, the PWM duty cycle may be 658 AC cycles out of 1,000 AC cycles, with the rest of the 1,000 AC cycles being turn off.
In step 112, hardware 132 outputs a PWM signal, i.e., a periodic series of pulses, to trigger a solid state relay 134 (SSR 134) to periodically turn on for the ascertained number of the ‘on’ AC cycles, e.g., 658 AC cycles out of 1,000 AC cycles. The 658 “on” cycles may be temporally distributed throughout the 1000 AC cycles to ensure smooth power delivery throughout the 1000 cycles of the example.
Consequently, in step 114, the AC power source is controlled by SSR 134 to periodically supply power to heater 135 for the ascertained number of the ‘on’ AC cycles, e.g., 658 AC cycles out of 1,000 AC cycles.
In step 116, the load temperature 136 is measured, and an iteration of the example conventional method is completed. Subsequently, feedback loop 138 feeds load temperature 136 into summing function 124 for the next iteration of the example conventional method.
As can be appreciated from the above description, if a resolution of R is required, then an iteration of the example conventional method will typically require a time interval of 1/R AC cycles. The time interval represents a response time for updating the duty cycle command. Typically, since R is much less than 1, the time interval, or response time, represents multiple AC cycles. For example, if R=0.1%, the response will be 1,000 AC cycles, or 16.67 seconds.
FIG. 2 illustrates output signals of the example conventional method and the example conventional system illustrated in FIGS. 1A and 1B.
Signal 210 illustrates the AC line voltage of the AC power source over time. Typically, the AC power source may have an AC frequency of about 60 Hz. Accordingly, each AC cycle (denoted by ‘p’) represents approximately 16.67 milliseconds (ms).
Signal 220 illustrates the quantized duty cycle command. Signal 230 shows the PWM signal generated by hardware 132, which is used as an input into SSR circuit 134. The output of SSR circuit 134 is shown by signal 240.
As shown in the example of chart 220, the quantized duty cycle command associated with signal 220 remains constant for a time interval of 1,000 AC cycles, or 16.67 seconds, given the AC frequency of 60 Hz. This is because it requires 1,000 AC cycles to complete an iteration of the example conventional method, given the required resolution of 0.1% in the example. However, a response time of 16.67 seconds for updating the duty cycle command may be disadvantageously too long and therefore unacceptable for many manufacturing processes such as wafer processing, which may require a response time of, for example, less 5 seconds with a 0.1% resolution or even finer resolutions.
Furthermore, iterations of the prior art method generally contain the same pattern of “on” pulses and “off” pulses, resulting in a periodically repeating pattern of “on” cycles and “off” cycles for power delivery to the heater. Such periodicity, particularly since they occur over a relatively long period of time, often times negatively affect substrate processing, as it may result in larger peak deviations from the desired setpoint, rendering it more difficult to achieve sufficiently tight process control results from wafer to wafer.